Oxidation with a “Stopover” – Stable Zwitterions as Intermediates in the Oxidation of α-Tocopherol (Vitamin E) Model Compounds to their Corresponding ortho-Quinone Methides

As a prominent member of the vitamin E group, α-tocopherol is an important lipophilic antioxidant. It has a special oxidation chemistry that involves phenoxyl radicals, quinones and quinone methides. During the oxidation to the ortho-quinone methide, an intermediary zwitterion is formed. This aromatic intermediate turns into the quinone methide by simply rotating the initially oxidized, exocyclic methyl group into the molecule's plane. This initial zwitterionic intermediate and the quinone methide are not resonance structures but individual species, whose distinct electronic structures are separated by a mere 90° bond rotation. In this work, we hindered this crucial rotation, by substituting the affected methyl group with alkyl or phenyl groups. The alkyl groups slowed down the conversion to the quinone methide by 18-times, while the phenyl substituents, which additionally stabilize the zwitterion electronically, completely halted the conversion to the quinone methide at −78 °C, allowing for the first time the direct observation of a tocopherol-derived zwitterion. Employing a ¹³C-labeled model, the individual steps of the oxidation sequence could be observed directly by NMR, and the activation energy for the rotation could be estimated to be approximately 2.8 kcal/mol. Reaction rates were solvent dependent, with polar solvents exerting a stabilizing effect on the zwitterion. The observed effects confirmed the central relevance of the rotation step in the change from the aromatic to the quinoid state and allowed a more detailed examination of the oxidation behavior of tocopherol. The concept that a simple bond rotation can be used to switch between an aromatic and an anti-aromatic structure could find its use in molecular switches or molecular engines, driven by the specific absorption of external energy.     

Stable meso‐Aryl β‐Alkyl Hybrid Sapphyrin with a Warped π‐Conjugation Circuit and Neo‐Confused Sapphyrin–Silver(I) Complex

A meso‐aryl and β‐alkyl substituted sapphyrin and its rhodium(I) and silver complexes were synthesized. This sapphyrin was so stable that the non‐inverted and warped structure could be analyzed by X‐ray crystallography even in its neutral state. Its bis‐rhodium(I) complex has a more planar structure than the sapphyrin with enhanced aromaticity over the conjugation circuit. On the other hand, silver metalation of the sapphyrin caused a marked core rearrangement into a neo‐confused sapphyrin derivative with a C(α)?N bond and a twisted macrocycle.     

Nature of Bonding in Bowl‐Like B36 Cluster Revisited: Concentric (6π+18π) Double Aromaticity and Reason for the Preference of a Hexagonal Hole in a Central Location

The bowl‐shaped C_6v B₃₆ cluster with a central hexagon hole is considered an ideal molecular model for low‐dimensional boron‐based nanosystems. Owing to the electron deficiency of boron, chemical bonding in the B₃₆ cluster is intriguing, complicated, and has remained elusive despite a couple of papers in the literature. Herein, a bonding analysis is given through canonical molecular orbitals (CMOs) and adaptive natural density partitioning (AdNDP), further aided by natural bond orbital (NBO) analysis and orbital composition calculations. The concerted computational data establish the idea of concentric double π aromaticity for the B₃₆ cluster, with inner 6π and outer 18π electron counting, which both conform to the (4n+2) Hückel rule. The updated bonding picture differs from existing knowledge of the system. A refined bonding model is also proposed for coronene, of which the B₃₆ cluster is an inorganic analogue. It is further shown that concentric double π aromaticity in the B₃₆ cluster is retained and spatially fixed, irrespective of the migration of the hexagonal hole; the latter process changes the system energetically. The hexagonal hole is a destabilizing factor for σ/π CMOs. The central hexagon hole affects substantially fewer CMOs, thus making the bowl‐shaped C_6v B₃₆ cluster the global minimum.     

meso‐3,5‐Bis(trifluoromethyl)phenyl‐Substituted Expanded Porphyrins: Synthesis,Characterization, and Optical,Electrochemical, and Photophysical Properties

Trifluoroacetic acid‐catalyzed condensation of pyrrole with electron‐deficient and sterically hindered 3,5‐bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso‐3,5‐bis(trifluoromethyl)phenyl‐substituted expanded porphyrins including [22]sapphyrin 2 , N‐fused [22]pentaphyrin 3 , [26]hexaphyrin 4 , and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20‐tetrakis(3,5‐bis(trifluoromethyl)phenyl)porphyrin 1 . These expanded porphyrins are characterized by mass spectrometry, ¹H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO–LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.     

Aromaticity and Diatropicity of Polyacenequinododimethides

Typical polyacenequinododimethides exist only in a single classical structure. These hydrocarbons are moderately aromatic and diatropic, although they have no aromatic conjugated circuits. This apparent dichotomy was resolved with our graph theory of aromaticity and magnetotropicity. Many nonconjugated circuits were found to contribute collectively to aromaticity and diatropicity. For individual molecules, local aromaticity increases with distance from the exo‐methylene groups. This fact indicates that the conjugated‐circuit model is not always applicable to semibenzenoid hydrocarbons such as polyacenequinododimethides.     

Cyclobutadiene dianion derivatives – Planar 4c,6e or three-dimensional 6c,6e aromaticity?

The spatial magnetic properties (Through Space NMR Shieldings – TSNMRS) of two cyclobutadiene derivatives (2 and 5) and of a number of cyclobutadiene dianion derivatives (3, 4 and 6–8) have been calculated by the GIAO perturbation method employing the Nucleus-Independent Chemical Shift (NICS) concept of P. v. Ragué Schleyer, and visualized as Iso-Chemical-Shielding Surfaces (ICSS) of various size and direction. TSNMRS values can be successfully employed to quantify and visualize the (anti)aromaticity of the compounds studied and to discuss the influence of Li⁺ complexation to cyclobutadiene dianion (4a, 7 and 8) on planar 4c,6e or three-dimensional 6c,6e aromaticity.     

芳环超分子体系中的π-π作用

π-π作用是芳环超分子体系中广泛存在的一种重要的弱相互作用. 本文对π-π作用的特征、形式、有效参数和表征方法等作了比较全面的综述, 同时小结了影响π-π作用的一些重要因素, 最后还概述了近年来π-π作用在超分子组装、不对称催化、有机半导体材料等领域的研究进展.     

Sandwich Complexes of the As42-Aromatic Ring with Some Transition Metals

The equilibrium geometries, energies, harmonic vibrational frequencies, and nuc- leus independent chemical shifts (NICS) of the new type sandwich structures [As4MAs4]n- (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt; n = 0, 1 or 2) are investigated at the B3LYP level.All the [As4MAs4]n- species adopt staggered (D4d) conformations as their stable structures and eclipsed (D4h) conformations as their transition states, and once the sandwich complexes are formed, the As42- square properties remain unchanged.The NICS calculation confirms that the complexes of Fe, Co, and Ni are aromatic with negative NICS values, and those of Ru, Rh, and Ir exhibit slight aromaticity, while those of Pd, Os, and Pt show slight antiaromaticity.